1. Field of the Invention
The present invention relates to fabrication of semiconductor devices, and more particularly, to a method of forming a silicate based dielectric on a silicon substrate and a semiconductor device formed thereby.
2. Description of the Related Art
In semiconductor devices such as complementary metal oxide semiconductor (CMOS) devices, a silicon dioxide (SiO2) based gate dielectric is formed on a silicon substrate. With increased miniaturization of transistors in CMOS devices, there has been an impetus to replace SiO2 as the gate dielectric with alternative material that has a higher dielectric constant. This is because the feasibility of CMOS scaling is increasingly dependent on the gate dielectric.
To increase the integration density of semiconductor devices, the thickness of SiO2 layers has been reduced. However, the reduction of the thickness of SiO2 layers causes a problem of diminishing reliability of the SiO2 layers as gate dielectrics. Evidence of diminished reliability for SiO2 layers thinner than 25 Å has been reported, and more recently revised to 16 Å. More recent assessments indicate diminished reliability for thickness below 16 Å.
Further reduction of the thickness of SiO2 layers would eventually require replacement of the SiO2 layers with some alternative material with a higher dielectric constant. An approach pursued by numerous groups is to create a dielectric stack, with an interfacial layer of SiO2 and an overlayer of a high permittivity dielectric, such as a metal oxide or silicate. Although the interface between silicon substrate and SiO2 layer is known to have good stability, this approach still has such drawbacks as the SiO2 layer limits the capacitance of the stack and fabrication of the stack on an atomic length scale.
Thus, alternative dielectrics are highly desirable if they can provide an equivalent capacitance with a reduced leakage current, taking account of numerous other considerations such as compatibility with silicon, thermal robustness, and ease of integration and deposition. A practical limit on the capacitance of a metal-oxide or a silicate on silicon structure is posed by the occurrence of an interfacial layer of SiO2. The interfacial layer may arise from substrate oxidation during dielectric growth, or it may be caused by post-growth oxidation, or it could even be a remnant from a preexisting SiO2 layer.
Fabrication of a semiconductor stack formed of metal-oxide and silicon layers adjacent to each other is severely hampered by the unfortunate fact that most metal-oxides are highly permeable to oxygen. If deposition of a metal-oxide layer is performed at elevated temperatures and in an oxygen rich ambient, excess oxygen can diffuse through the metal-oxide dielectric layer and react with the silicon substrate. For example, when a compound such as zirconium (Zr) oxide or hafnium (Hf) oxide is deposited on hydrofluoric (HF) etched silicon surfaces, there is generally formed an interfacial SiO2 layer having the thickness of 5-15 Å. Metals such as lanthanum (La) and yttrium (Y), which easily form silicates, develop both an interfacial SiO2 layer and a significant level of Si (e.g., 10-15 at %), which may further reduce the capacitance of the stack. Further, once the metal-oxide layer is deposited, post-growth oxidation may be needed to reduce the leakage current, which will also reduce the capacitance of the stack.
It is well known that when SiO2 is heated in the ultrahigh vacuum (UHV), it is converted into SiO which is volatile. Thus, SiO2 formed on a substrate can be removed from the substrate by thermal treatments in the UHV circumstances. Furthermore, the reaction is known to be inhomogeneous, starting with pinholes that gradually merge. The reaction takes place at temperatures ranging from 800° C. to 1050° C., depending on the thickness of the SiO2 layer.
However, although most metal-oxides are highly permeable to oxygen, the removal of the SiO2 layer formed on the Si substrate during a metal-oxide deposition has not been successful. For example, investigation of a film formed of layers of “ZrO2/SiO2/Si(001)” shows that at temperatures between 900° C. and 1000° C., the film is converted into islands of Zr silicide. Another investigation of a film where Al2O3 layer is deposited on a Si substrate shows a different pathway to dissociation, that is, volatilization of the metal and conversion of the surface to atomically clean, albeit rough, Si(100). In other investigations as well as the above examples, it is shown that oxygen loss is accompanied by the catastrophic destruction of the dielectric.
In addition to the above-mentioned approaches to solve the SiO2 layer problem, there has been provided a method of minimizing Si-intermixing between a metal oxide layer and a Si substrate. This method is described in “Yttrium Silicate Formation on Silicon: Effect of Silicon Pre-oxidation and Nitridation on Interface Reaction Kinetics”, by Chambers et al., October 2000, Applied Physics Letters, Vol. 77, No. 15, pp. 2385-2387. Chambers et al. describe a method for minimizing Si-intermixing by introducing barrier layers of nitrided silicon interfacial layer between a metal-oxide layer and a Si substrate. However, Chambers et al. have not provided any information regarding the removal of SiO2 formed by depositing the metal-oxide on the Si substrate.
Therefore, a need exists for a method of forming a dielectric stack having a dielectric layer which replaces a conventional SiO2-based dielectric layer. Further, it will be advantageous to provide a method of forming a dielectric stack which has characteristics of increased capacitance and reduced leakage current.